In general, a motor is provided in a compressor which is a mechanical apparatus for receiving power from a power generation apparatus, such as an electric motor, a turbine, etc. and compressing the air, refrigerant or other various operating gases to raise a pressure. The motor has been widely used in electric home appliances such as refrigerators, air conditioners, etc., and its application has been expanded to the whole industry.
In particular, the compressors are roughly classified into a reciprocating compressor in which a compression space for sucking and discharging an operating gas is defined between a piston and a cylinder so that the piston can be linearly reciprocated in the cylinder to compress a refrigerant, a rotary compressor in which a compression space for sucking and discharging an operating gas is defined between an eccentrically-rotated roller and a cylinder so that the roller can be eccentrically rotated along the inner wall of the cylinder to compress a refrigerant, and a scroll compressor in which a compression space for sucking and discharging an operating gas is defined between an orbiting scroll and a fixed scroll so that the orbiting scroll can be rotated along the fixed scroll to compress a refrigerant.
Recently, a linear compressor which not only improves a compression efficiency but also has a simple structure has been actively developed among the reciprocating compressors. In particular, the linear compressor does not have a mechanical loss caused by a motion conversion since a piston is directly connected to a linearly-reciprocating driving motor.
FIG. 1 is a block diagram of a motor control device used in a conventional linear compressor.
As illustrated in FIG. 1, the motor control device includes a rectification unit having a diode bridge 11 receiving, rectifying and outputting AC power which is commercial power and a capacitor Cl smoothing the rectified voltage, an inverter unit 12 receiving a DC voltage, converting the DC voltage to an AC voltage according to a control signal from a control unit 17, and supplying the AC voltage to a motor unit, the motor unit having a motor 13 and a capacitor C2 connected in series to the motor 13, a voltage sensing unit 14 sensing a both-end voltage of the capacitor C1, a current sensing unit 15 sensing a current flowing through the motor unit, an operation unit 16 operating a counter electromotive force (EMF) from the voltage sensed by the voltage sensing unit 14 and the current sensed by the current sensing unit 15, and the control unit 17 generating a control signal by reflecting the counter EMF from the operation unit 16 and the current sensed by the current sensing unit 15.
In the conventional linear compressor shown in FIG. 1, additional costs and space are needed because the high-capacity capacitor C2 connected in series to the motor 13 is provided in the linear compressor. In addition, although the cooling capacity variability characteristics based on the load are determined by the capacity of the capacitor C2, in the prior art, it is not easy to change the capacity of the capacitor C2. Moreover, the preparation and selective connection of a plurality of capacitors cause difficulties in terms of cost, space, and design.
FIG. 2 is a graph showing changes of a stroke and an input voltage of the motor of FIG. 1. In the conventional linear compressor, if the capacitor C2 is removed in a simple manner, as shown in FIG. 2, a phenomenon in which a voltage applied to the motor is reduced, i.e., a jump phenomenon occurs near the top dead center (TDC), so that the cooling capacity variability (under stroke operation) is impossible. In the graph of FIG. 2, the closer to 0.00, the closer to the TDC.
It is also essential to precisely set an initial current value so as to calculate the counter EMF or voltage by integrating the current from the current sensing unit which senses the current flowing through the motor unit.
FIG. 3 is a graph showing a conventional current integration curve. As shown in FIG. 3, initial current values at the peak of the current i can be set as points A, B, and C. Here, point C corresponds to the actual peak of the current i, point B has a smaller value than point C, and point A has a smaller value than point B.
Consequently, when a voltage Va graph in which point A has been set as a peak, a voltage Vb graph in which point B has been set as a peak, and a voltage Vc graph in which point C has been set as a peak are compared with one another, the integrated values have the highest peak in the voltage Va graph, the second highest peak in the voltage Vb graph, and the lowest peak in the voltage Vc graph. That is, how the initial value at the current peak is set makes a significant difference in the integrated voltage. Accordingly, if the initial value at the current peak is not appropriate, the integrated current values are not suitable for the use in precise control because offset values are continuously accumulated.